Telephone call diverting system



Aug. 24, 1943.

TELEPHONE CALL DIVERTING SYSTEM Filed March 31, 1942 CON/V.

E. L. ERWIIN 2,327,877

2 Sheets-Sheet 1 INVENTOR ELERWl/V 8V v ATTOR/VEV Aug.24,1 943. 'E. L.ERWIN 2,327,877

TELEPHONE CALL DIVERTING SYSTEM Filed March 31, 1942 2 Sheets-Sheet 2 xlNl EN TOR I By ELERWl/V ATTORNEY Patented Aug. 24, 1943 TELEPHONE CALLDIVERTING SYSTEM Edson L. Erwin, Towaco, N. J., assignor to BellTelephone Laboratories, Incorporated, New

York, N Y., a. corporation of New York Application March 31, 1942,Serial No. 437,007

9 Claims.

This invention relates to dial private branch exchange telephone systemsin which automatic switches are employed for establishing connectionsand more particularly wherein automatic switches at an automatic centralofiice are con-' trolled over a trunk circuit by a subscriber of thprivate branch exchange. An object-oi the inventionis" to preventsubscribers of the private branch exchange from establishing, bydialing, connections over the trunk to certain points in the centraloffice, as for example a toll operator and'to automatically divert suchcalls to' an attendant at the private branch" exchange-and'morespecifically, the ob-- ject is to prevent the calling subscriber fromcircumventing such a diversion by irregular manipulation of his dial orswitchhook to obtain an unauthorized connection.

Systems have heretofore been employed in which an attempt by subscribersin a dial private branch exchange to obtain a connection to a tolloperator results in' the transfer of the calling subscribers linefromthetrunk to an attendant at the private branch exchange which iseffected by an arrangement employing an auxiliary autor'natic "switchassociated with the trunk at the private branch exchange which switch isactuated. by the dialing pulses transmitted to the trunk, in such amanner that when a certain code is'transmitted, zero for example,thecalling subscribers line is disconnected from the trunk and connectedto an attendants position at the private branch exchange. Such a systemisdisclosed and claimed in United States Patent switches may take such aposition as to permit the undesired connection, While the auxiliaryswitch having a different setting does not recognize this fact and hencedoes not cause the call to be transferred to the attendant. I

Further it has been known, as disclosed, for example, in United StatesPatent 2,040,291, issued .May 12, 1936, to F. A. Bonomi, to prevent aprivate branch exchange subscriber from obtaining unauthorized tollservice which is accomplished by a current reversal back over the trunl;to the private branch exchange which reversal is initiated by theresponse of the toll operator, which reversalis efiective to cause thecalling line to be "transferred to the attendant.

A feature of the present invention resides in means whereby if thepulses transmitted to the trunk occur at a rate per second above apredetermined maximum (12 for example) every other pulse willbe maskedor absorbed and only alter: nate pulses will be appliedto the auxiliaryswitch and transmitted to the centra11 office switches, which alternatepulses will occur at such' a slow rate that both switches should operatein sub-, stantial synchronism and although such pulses Will result in awrong number? setting of the central ofiice switches, ifsuchsettingeffects a connection to one of the points to which thecalling subscriber is not entitled, the auxiliary switch will bestanding in a' corresponding'position and will effect the desiredtransfer ofthe calling line to the attendant;

Another feature'resides in locating the auxiliary switch and itsassociated equipmentand circuits 'at the central ofiice end of the trunkinstead of at the private branch exchange, whereby the possibility ofnon-synchronous. operation of the auxiliary and central ofiice switchesis re-' duced.

Another feature, whereby the transfer of the calling line to theattendant is effected, resides in means controlled by the auxiliaryswitchto reverse the polarity of the central ofiice battery supplied tothe private branch exchange over the trunkr 1 Another feature resides inmeans for eiiecting a so-called' interdigit time control whereby if thetime interval between successive digits dialed is less than apredetermined minimum (.6 second, for example) a current reversal overthe trunk will be effected and the calling line transferredto theattendant.

A further feature'resides in means for correcting the pulses transmittedover the trunk and for repeating them to control both the auxiliaryautomatic switch and the central ofiice switches. The operation of thecircuits and arrangement of the invention embodying the foregoingfeatures, will be understood from the following description andaccompanying drawings in which:

that portion of Fig. 1 to the left of the brokenline X represents a dialprivate branch exchange including a subscribers stationA, automaticswitching equipment including i a selector corinect'orSC, an attendantsposition 0', a cenconnector SC and means including a polarized relay Pin series with the ring conductor of the trunk, efiective when operatedin one direction, 1. e., by a current reversal, to transfer the-line ofthe subscriber A from the trunk RT to the attendants position, and thatportion of Fig. l to the right of the broken line XX, and Fig. 2,represents the equipment associated with trunk RT at the central ofiiceend thereof.

Referring to the drawings it will be assumed that it is not desired topermit the subscribers of the private branch exchange to dial certainjoints through the central ofiice, a toll operator.

for example, over the dial trunks connecting the private branch exchangewith the central office,

while at the same time permitting the subscribers to dial local stationsof the central office without hindrance and therefore the trunks to thecentral ofiice are arranged in accordance with the presentinvention, i.e., are arranged to. permit calls tolocal stations ofthe central ofiiceb'ut automatically cause a current reversal back over the trunk to causethe. calling station line to be disconnected from the trunk andtransferred to the attendant s switchboard when thelstation dialscertain codes; such forex'ample as 0, 110, or 211, which codes arefrequently used for calling; a toll operator. It will further'be assumedthat the subscriber at station Adialslthe designated code to'select acentral office trunk andthen dials the designated. code foraitolloperato'r, which may'beifiz, 110, or 211.

When the subscriber removeshis receiver and actuateshis dial linaccordance with the predetermined trunk-code, the automatic equipment atthe privatebranch exchange .(not shown) causes the brushes of theselector connector SC to engage thexbank' contacts to. which centraloffice trunk RT is connected, whereupon ground is temporarily connectedto. the ring conductor I of the trunk in a manner described, in line 35et seq. on page 2 of United States Patent 1,868,326 to G. V.King,'issued- July- '1'9, 1932; whereupon relay 2 at the distant end .ofthe: trunk operates in the followingcircuit, ring-fconductor of thetrunk; lower left winding ofrepeatingcoil 3, contacts4of relay 5',contacts 6' of relay 1-, andwinding of relay 2 to battery. Relay 2operates and completes a circuit including contacts 8 of relay 1:cooperate. relay 9:which relay, at its contacts l0, connects ground: tothe sleeve conductor of the central onic'e connector CON; locks, at itscontacts I2 under control of relay l3,-and closes at its contacts [4 acircuit to' operate polarized relay-l5 over its; upper winding, whichcircuit includes resistance. is, of the; order .015 10,00, ohms.

Relay l5 operatesv and; closes its contacts to operate relay ll. whichrelay in operating closes ate'sin series with the high resistance?!thereby closing its contacts 28 to operaterelay 1, opens the lockingcircuit of relay 3,-and by closing its contacts 29'locks relay 2;operated. Relay 9, however, remains operated over its-initial circuit.

Operation. of" relayl first. closes, t contacts 30 r tral office trunkRT accessible to the selector and 3| to connect tip and ring conductorsof the trunk from the private branch exchange, to

ground and battery respectively in series with the lower and upperwindings of relay l5. Relay 1 continues to attract its outer armaturethus opening its contacts 6 to interrupt the original energizing circuitfor relay 2, closes its contacts 32 to operate relay 33,

and opens itsv contacts 8 to release relay 9. a 7

loop from the private branch exchange and responds to the dial pulses.

When relay ll operated in response to the initial operation of relay l5,it closed, the trunk loop to the central ofllce over contacts 2|. and 23of relays 22 and 2 4,r'e-

spectively, and when the line finder found the line thus-causing. theoperation of relay 1, this operationof relayl'5, conductor38,'contacts=39. oiv a relay 35', contacts of relay M, and winding ofrelay 3.5,to battery. Relay35 in operating closes its contacts 42thereby completing a locking cir-. cuit for itself independent ofcontacts 3110i relay llzwhich :circuit can be traced from battery,

windingof relay 35,-contacts' 40 of relay M, contacts 42 of relay 315;,and contacts 34 of relay 1 to ground.

Relay 35 also closes its contacts thereby shortcircuiting, contacts, |;8.,'.of relay I! to maintain closure of the loop. over contacts 2| and 23of relays Z-Zjand 24, respectively. On the -first in'-. terruptionof thetrunk loop at the dial contacts of the. calling;station, relay-l5releases therebyreleasing relay 1'! which; closes its contacts, 44 tooperate relay 45, the circuit for. which canbe ace ro ro nd cont s 3 frela c nuc con acts 4. rot-r y -,-c nta s of relay 41 contacts 4,80?relay 49,- and'winding-of relay 4.5 to b tt ry; --Rc ay in o eratinlocks at its contacts 50 under control ofrelay 4 9. Clor f nta s :Qt e1a5 p r t u l rl n c o ur f nta sjfi "of this. same re (45 closes, acircuittofcperate relay 54 which can be traced from ground,-outer leftcontacts of relay 2, contacts 55-.of relay 1, contacts 560! relay 4|,conductor 51, contacts 53 ofirelay 45, and winding of relay 541tobattery;- -'Closure of contacts 58 of relay 24; and contacts 5,9 orrelay; 5.4 close parallel operating paths for relay. 60.

Closure ofcontacts 6,! ofrelay 54 completes-"a;

' circuit to operate relay 4'!- which can be traced from ground,contacts 61 of relay54; conductor 62 contacts 63- of.'. relay 4|,conductor '64, and.

winding of relay 41' to batterywhi'ch latter relay closes its contacts65 to lock itselfunder control x i Further, I closure ofcontacts'iit'ofrelay 54 -cz aguses-opera of the back contacts- 44' of vrelay 'll.

tion of steppinglmag net E1 of auxiliary switch" which circuit 'can betracedfrom ground on conductor 57', over contacts 6,6 cf relayi 54,:to'batte'ry in. series 'withmagnet G ll" Switch lit-is of the typewhichstepson-the release of its rotary 'magmal position at this time;

Relay E l in operating opens net. and therefore .it, doesnot moye'iromits norclosure'efiected whenrelay lloperated, but, operits". contacts 23thereby introducing a, break in the original loop relay 22 and thereforethe operation of'relay 24 is ineffective to-open the loop.

When relay 49 operates, however, it opens its contacts 48 to deenergizethe slow-to-release relay 45. When relay 45 finally releases it opensits contacts 69 thereby opening the loop to the central oflice therebycausing a stepping movement of the switches thereat. Prior to therelease of relay 45, relay 49 is held operated over contacts of relay 45which relay (49) closes its left-hand contact thereby short-circuitingrelay 22 and insuring its released condition butwhen relay 45 operatesthis short circuit is removed and relay 22 operates in series with relay49 in a circuit from ground on conductor 35, contacts 19 of relay I1,winding of relay. 22, lefthand contacts and windingof relay 49 tobattery. Relay 49 remains operated in this circuit.

The release of relay 45 opens the circuits of relays 54 and 24 whichlatter relay, however, is slow to release. Relay 54 in releasingopensits contacts E5 to deenergize the rotary magnet 51 thereby causing theswitch 68 to move into its first position. Relay 24 in releasingrecloses its contacts 23 but due to its slow-to-release characteristicthis does not occur until relay 22 has operated whereby the pulsingloo-p to the central ofnce remains open at contacts 2! of relay 22.

It willnow be noted that auxiliary switch 68 and the centralofiiceselector switch have both moved in substantial synchronism into theirfirst positions. When relay I1 reoperates at the end of the first pulse,relays 22, 49 and 41 release and prepare the circuit for the next pulse,'i. e., the release of relay H which release reoperates relay 45. Asbefore mentioned relays 45 and 24 are slow in releasing and under normalpulsing, i. e., below l2 pulses per second, relay 24 is not fullyreleased to close its contacts 23 until relays 22 and 49 are released bythe operation of relay ll. Therefore relay 24 controls the length. ofthe pulse intervalat normal speeds. When relay ll releases for the nextpulse relay 45 reoperates as before provided relay 4'! hasreleased'thereby again closing the loop at its contacts 69. If, however,relay 41 has not released when the contacts 44 of relay I! close relay41 remains locked and the circuit of relay 45 is open at con tacts 45 ofrelay 4'! and therefore relay-45 does not operate to close the loop andtherefore acts to insure a minimum interval during which the loop isopen, at dial speed which is slightly higher than normal.

The arrangement just described is substantially similar to the impulsecorrecting arrangement disclosed in United States Patent 2,249,854issued July 22, 1941, to T. C. Rehm, in which relays I3, [4 and i5 ofboth figures, and relay H of Fig. 2, respectively correspond to relays45, 24, 49 and 22 of the present application.

Cancellation of alternate pulses If the pulses occur at a considerablyfaster than normal rate, i. e., l2 or more pulses per second, relay 24which is slow to release will not release until after relay 45 isreleased and therefore relay 45 controls the length of the closed loopinterval.

At this speed relay 4'! will not release between the successive openingsof contacts 44 of relay I! and hence only alternate pulses will betransmitted to the central office and to the switch 68 which will occurat such a slow rate that there will be no question of these switchesoperating Interdz'gz't time control When relay fifl operated as beforedescribed it closed its contacts H to operate relay 12 which in turnoperates relay .13. An alternate energizing circuit for this latterrelay was also closed when relay 22 operated. Relay 5G is slow torelease and therefore holds operated between pulses.

At the end of the first digit if the interval between the last pulse ofthe first digit and the first pulse of the second digit is greater thana predetermined interval (six-tenths of a second, for example) relay 5!]will release due to the release of relays 24 and 54 and opens thelocking circuit of relay 12 which is independent of relay l3. Relay 12will therefore release and in turn release relay 13 provided relay 22has released. Relay 13, however, is also slow to release and if thenextor second digit is'dialed too soon and relay 60 reoperates beforerelay l3 had fully released, a circuit is completed to operate reversingre1ay 5 in a circuit as follows: ground, contacts ll of relay '52,contacts '35 of relay l3, conductor l6 and right-hand winding of relay 5to battery. Relay 5 in operating opens its contacts 4 and 26 and closescontacts Ti and 18 thereby reversing the polarity of the batterysupplied'back over the trunk RT to the private branch exchange where itcauses operation of polarized relay P in the ring conductor of thetrunk. Operation of relay P operates relay 19 which disconnects the lineof the calling subscriber from the trunk RT and connects it 'to theattendants position 0 in the manner shown in United States Patent2,040,291 to F. A; Bonomi, issued May 12, 1936. The attendant thereupontalks with the subscriber to ascertain what number he hasdialed. It willtherefore be noted that relays 54, 5! l2 and 13 are effective to divertthe calling line to the private branch exchange attendant if theinterval between successive digits is less than a predetermined minimum.7

First digit 1 When the selector 5%! has stepped to the first terminal asthe result of a single pulse and relay 60 releases, ground is connectedat contacts 89 of relay 65 to operate the release relay 8! of the switchto restore it to normal. This circuit can be traced from ground,contacts 82 of relay 35, conductor 33, contacts of relay 55, brush andthe first terminal of switch 58, contacts 84 of relay 85, contacts 86 ofthe off-normal switch ON and winding of the release magnet 8! to batterythereby releasing the switch 68 and restoring it to normal.

The disclosure of the present invention shows an arrangement forpreventing the subscriber from Obtaining a connection to an operator atthe central oflice whose code may be 110, 211, or 0, depending on thetype of central ofice in which the trunk terminates and we will nowassume that the subscriber has dialed the code which, if not interferedwith, will call in a toll operator, access to which operator thesubscriber is not entitled.

Dialing of the first digit 1 as before described causes the-switch 68 toadvance one step whereupon the release ma'gnet operates to restore theswitch and thuscancelling the digit. The second digit 1 again advancesthe switch 58 to the first .terminal whereupon the switch is. releasedant returned .to. normal as before; The third dig-it consistingoiiten'pulsescauses: the switch 68 to advance, step. by step,- to thetenth terminal whereupon when relay 60 releases, ground on conductor 83is connected over contactstll of relay Eli, the brushes 68, at its tenthterminal, contacts 81 of relay 85,-and conductor 16 to battery in serieswith the reversing relay 5 -to cause the calling line to be disconnectedfrom the trunk at the private branch exchange and transferred to theattendant as before described. Operation of relay 19"at the privatebranch exchange causes the trunk loop to be opened whereupon avdisconnect is given to the central ofiice equipment.

When the central ofiice switches restore the div verting equipmentrestores to normal.

First digit 2 If we assume that the central ofiice equipment is arrangedto call in a toll operator in response to the receipt of the code 211and that the subscriber dials this code, the first digit-,2 will causethe switchesto advance to the second terminal and the release of relay6!) will supply ground over thes'econd terminal of the switch andcontacts 88 of relay 8'5 to operaterelay Bit-which relay in operatingcloses its contacts 90 to operate the release magnet 8| in a circuitwhich can be traced from ground, contacts 98 of relay 89, contacts 9| ofrelay'85 and contacts 36 of the offnormal switch ON to the releasemagnet. Operation of'therelease magnet restores the switch 58 to normal.Operation of relay 89 also closes its contacts 92 to lock itself toground under control of relay 35. Relay 8!! also closes its contacts 93to operate relay 85 which is thereafter also held under control of relay'35. When the second digit '1 is dialed the selector 68 advances asbefore to the firstterminaland when relay Bll'releases a circuit iscompleted to operate the reverse relay 5 over contacts 94 of relay 85,now operating. Operation of relay 5 causes disconnection of the callingline from the trunk and its transfer to the attendant as before.

I First digit 0 The third code which the present invention is arrangedto intercept is zeroor ten pulses and if this digit is first dialed, theresult will be the same .as in the case when code 110 wasdialed, f

First digit 2 and second digit other than 1 In case a subscriber dials acode in which the firstdigit is 2 and the second digit some other numberthan 1, relays 89 and 85 will be operated at the completion of the firstdigit and the release of relay 6!! will connect a ground over the brushof switch 68 and whichever terminal it is standing on, other than 1, tooperate relay 4| when relay, I3 is released which occurs a shortinterval after relay releases. opens at its contacts 40 the energizingcircuit for relay 35 which releases and closes its contacts 95 toconnect ground over conductor 96 to operate the release magnet andrestore the selector switch Relay 4| holds over its right winding andcontacts 91. to ground at contacts 34 of relay 1. Relay 4l operated alsoopensits contacts 56 to prevent operation of relay 54 and any furthermove- Operation of relay All ment of the selector switch'68 bysubsequent'pulsing which, however, will be repeated, in the man'- 'n'erpreviously described, in response to the subscribers dial to control thecentral officeswitches.

First digit other than 1,2 or 0 If the subscriber dials for. the firstdigit of a code any number from 3 to 9, which will advance switch 68 toa corresponding terminal, which ter- 1 minals it will be noted areconnected together, the release of relay Ell will cause relay 4| tooperate and relay 35 to release thus releasing switch 68 and disablingit from further movement in re sponse to further dialing.

Disconnect The release of relay 1 releasesrelay 5, if operated, andreconnects' relay 2' to the ring conductor of the trunk. Relay 1 inreleasing reconnects ground at its contacts 98 to the tip of the trunkover the outer right contacts of relay 33 which relay operated whenrelay 1 operated.

Release ofrelay I also opens its contacts 32- thereby deenergizing relay33'but this relay is slow to release and holds ground on the'tip of thetrunk for a short interval after contacts 3I of relay 1 open.

When relay 33 finally opens its outer right contacts ground is removedfrom the tip of the trunk causing the equipment at the private branchexchange to return to normal in a wellknown manner.

From the foregoing description it will be observed that battery will bereversed back over the trunk to'the private branch exchange to call inthe attendant and disconnect the calling line from the trunk if thefirstcligit dialedis zero, if the code dialed is or 211, or if, due tomanipulation of the dial or switchhookjtrains. of pulses aretransmitted, which trains are separatedfrom each other by less than apredetermined time interval, six-tenths of asecond in the arrangementdisclosed.

Further, it will be noted that if pulses are transmitted which recur ata rate faster than the central ofiice switch can-accurately follow,

i. e., twelve or more per second, alternate pulses are masked orabsorbed and only every other pulse is permitted to control theauxiliary trunk.

switch and the central office equipment.

In other words, the auxiliary switch associated with the trunk followsand registers one or more of the first three digits dialed into thecentral ofiice. When the first digit registered on the auxiliary switchdoes not form a part of. a recognized toll code, i. e., 3 to 9,inclusive, further dialing is ignored and the call goes' through in anormal manner. If the first digit dialed.is:0, or 0 is preceded by 1,the call is diverted to the attendant. Further, if the first digit is 2andthe second digit is 1 the call will'also be diverted.

Further, the pulses transmitted to the central oflice are corrected andif the rate of recurrence.

is greater than the ability of thecentral ofilce switches to correctlyregister them every other 'pulse will be absorbed and therefore thepulses applied'to the auxiliary switch, and transmitted to the centraloflice, will be slow enough so that both the auxiliary switch and thecentral ofiice switches can operate in synchronism. Obviously, suchalternate pulsing will result in an improper setting of the centraloffice switches but if a code starts with 0, 10, 21, or 110 thecallingline will be diverted to the attendant and further control the centralofiice equipment prevented, if the dial at the station calling isretarded in such a manner that the digit dialed is split into two trainsof impulses which are too closely spaced, i. e., if the digits aredialed so as to succeed each other too rapidly, the'call is likewisediverted. i

It will, of course, be obvious that, by different connections of theterminals of auxiliary switch 68, operation of reversing relay can beeifected in response to other digit pulses, either singly or incombination.

What is claimed is:

1. In a telephone system, an automatic exchange, an operators positionthereat, a central office, a trunk line extending from said exchange tosaid central office and terminating in automatic switches thereat, asubscribers line terminating in said exchange adapted to transmit groupsof dial impulses to extend said line to said trunk and to control saidautomatic switches at the central office, means in said trunk at theexchange responsive to a reversal of current over said trunk todisconnect said line from the trunk and connect it to the operatorsposition, and other means in said trunk efiective in response to apredetermined minimum time interval between successive groups of dialimpulses transmitted to cause a current reversal to be transmitted oversaid trunk to said exchange.

In a telephone exchange system, an automatic exchange, an operatorsposition thereat, a central oiiice, a trunk line extending from saidexchange to said central cities and terminating in automatic switchesthereat, a subscribers line terminating in said exchange adapted totransmit groups of dial impulses to extend said line to said trunk andthereafter to control said automatic switches at the central office,means in said trunk for repeating said impulses to said central officeswitch, an auxiliary switch associated with and responsive to saidrepeating means adapted to advance in step with said central ofiiceswitches, means in said trunk at said exchange responsive to a reversalof current over the trunk to disconnect said line from the trunk andconnect it to the operators position, and means responsive to apredetermined setting of said auxiliary switch for reversing currentover said trunk, said repeating means being arranged to suppressalternate group impulses when said impulses recur faster than apredetermined rate whereby simultaneous advance of said central officeand auxiliary switches is insured.

3. In a telephone exchange system, an automatic exchange, an automaticcentral oifice, a trunk line therebetween, a subscribers lineterminating at said exchange, a subscribers station dial adapted totransmit groups of impulses over said line to connect the line to thetrunk and to thereafter transmit impulses over the trunk to the centralofiice, automatic switches at the central cifice, pulse repeating meansincluding a source of direct current associated with said trunk at thecentral office and adapted to control said central office switches inaccordance with the pulses transmitted over the trunk, and an auxiliarystepv-by-step' switch at the central oflice also controlled by saidrepeating means and arranged to move in substantial synchronism withsaid central office switches, and means responsive to a predeterminedsetting of said auxiliary switch by said repeating means for reversingthe direction of said direct'current source over said trunk to theexchange. I

4. In a telephone exchange system, an automatic. private branchexchange, an automatic central office, automatic switches thereat, atrunk line therebetween, a subscribers line terminating at saidexchange, means for transmitting impulses over said line to connect theline to the trunk and to thereafter transmitgroups of impulses vover thetrunk, pulse repeating means including a source of direct currentassociated with said trunk at the central office and adapted to controlsaid automatic switches in accordance with the pulses transmitted, anauxiliary step-by-step switch at the central oflflce also controlled bysaid repeating means and arranged to move in substantial synchronismwith said central oflice switches and to return to normal at the end ofeach group of pulses, and means responsive to predetermined first andsecond settings of said auxiliary switch in response to the first twogroups of impulses transmitted to reverse the connection of said currentsource to the trunk.

. 5. In a telephone exchange system, an automatic private branchexchange, an automatic central ofiice, automatic switches thereat, atrunk line therebetween, a subscriberfs line terminating at saidexchange, means for transmitting impulses over said line to connect theline to the trunk, and to thereafter transmit groups of impulses overthe trunk, pulses repeating means including a source of direct currentassociated with said trunk at the central office adapted to control saidautomatic switches in accordance with the pulses transmitted, anauxiliary step-by-step switch at the central office also controlled bysaid repeating means and arranged to move in substantial synchronismwith the central office switches and to return to normal at the end ofeach group of pulses, and means responsive to predetermined first,second and third settings of said auxiliary switch in response to thefirst three groups of impulses transmitted to reverse the connection ofsaid current source.

6. In a pulse repeater and corrector, an incoming line, an outgoingline, a relay associated with said incoming line responsive to impulsetrains transmitted thereover, means responsive to an initial operationof said relay for closing said outgoing line, means for opening saidoutgoing line eifective a predetermined time interval after the releaseof said relay to open said outgoing line, other means responsive to asucceeding reoperation of said relay to close said outgoing line andmeans efiective when the succeeding rate of operation and release ofsaid relay is above a predetermined maximum to prevent everyothersuccessive release of said relay from opening said outgoing line.

7. In a'pulse repeater and correotor, an incoming line, an outgoingline, a relay associated with said incoming line operated and releasedin accordance with current of impulses transmitted thereover, meansresponsive to operation of said relay to close said outgoing line, meansresponsive to release of said relay to open said outgoing line, andmeans responsive to the successive release and operation of said relayat a rate faster than a predetermined maximum to hold said outgoing lineclosed over every other release interval of said relay.

8. In an impulse repeater, an incoming line, an outgoing line, animpulse receiving'relay associated with said incoming line, means forrepeating to said outgoing line trains of impulses transmitted over saidincoming line and received by said receiving relay, said repeating meanscomprising a sloW-to-release relay for determining the minimum durationof said repeated impulses when and if the rate of the received impulsesis lower than a first critical rate, and also comprising afast-operating relay and a. slowcperating relay for determining theminimum duration of said repeated impulses when and if the rateof thereceived impulsesis equal to or in excess of said critical rate, andother means comprising a second slow-release. relay for preventing therepetition of every other received impulseif and when the receivedimpulses are equal to or in excess of a; second predetermined criticalrate higher than-said first critical rate.-

9. In a telephonesystem, an automatic exchange, an operatorspositionthereat, a. central oflice, a trunk line extending to saidexchange from said central ofiice and terminating in auto.- maticswitches thereat; a subscriber's line terminating in said exchangeadapted to transmit groups of dial impulses to extend said line to saidtrunk and to control, said automatic switches at the central ofiice,means in saidtrunkvat the exchange responsiveto a reversal of currentover lease relay, effective in response to a predetermined minimum time.interval between successive groups of dial impulses transmitted to causea. current reversal to be transmittedover said trunk to said exchange.

EDSON'L. ERWIN.

